The present invention generally relates to digital broadcasting systems. More particular, the present invention relates to frequency or time-frequency correlation-based synchronization for coherent Orthogonal Frequency Division Multiplexing (OFDM) receivers in a multi-carrier digital broadcasting system, such as Digital Video Broadcasting-Terrestrial (DVB-T), Digital Video Broadcasting-Handheld (DVB-H), and Integrated Service Digital Broadcasting-Terrestrial (ISDB-T) system.
OFDM transmission technique, being one kind of the multi-carrier modulation schemes, has been widely applied for modern high-data-rate digital communications and broadcasting due to its extreme efficacy on dealing with the multipath propagation effects. The OFDM technique has been adopted by several broadcasting systems such as Digital Audio Broadcasting (DAB), DVB-T, DVB-H and ISDB-T, and, moreover, by local area networks such as the HiperLAN/2 and IEEE 802.11a/g/n. Specifically, the (inverse) fast Fourier transform (FFT) technique is employed in an OFDM transmission system for efficiently implementing multi-carrier modulation and demodulation.
For coherent OFDM-based systems such as the DVB-T/H and ISDB-T systems, certain scattered pilots (known as SPs hereinafter) regularly posited in time- and frequency-dimensions are transmitted with predetermined known values together with information data at OFDM transmitters' end and used for channel estimation and equalization at OFDM receivers' end. Referring to FIG. 1, a diagram illustrating positions of SPs defined in DVB-T/H systems with respect to the time-frequency dimension in the frequency domain is provided. The positions of SPs in DVB-T/H systems can be expressed as follows:
For the OFDM symbol of index l (ranging from 0 to 67), carriers for which index k belongs to the subset {k=Kmin+3×(l mod 4)+12p|p integer, p≧0, kε[Kmin, Kmax]} are SPs, wherein p is an integer that takes all possible values greater than or equal to zero, provided that the resulting value for k does not exceed the valid range [Kmin, Kmax]. Kmax is 1704 for the 2K mode, 3408 for the 4K mode and 6816 for the 8K mode as defined by DVB-T/H standards.
The positions of the SPs should be detected and identified by means of a synchronization sequence (or synchronization procedure) at a coherent OFDM receiver. Assume that the received Radio Frequency (RF) signal is first down converted to the baseband using a tuner and a carrier recovery loop. A typical DVB-T/H baseband synchronization sequence 20 is illustrated in FIG. 2. After the start-up, pre-FFT synchronization is performed in step 21 in which all metrics are derived in time-domain from guard interval correlation. The baseband signal is then transformed to the frequency-domain through FFT. Subsequently, post-FFT synchronization is performed in frequency-domain in step 22 based on correlating the Continual Pilots (CP) of two consecutive OFDM symbols. Specifically, the pre-FFT and post-FFT synchronization blocks perform the sampling clock, OFDM symbol timing and carrier frequency synchronization.
After sampling clock, OFDM symbol timing and carrier frequency synchronization have been achieved via the pre-FFT and post-FFT synchronization, the positions of the SPs within an OFDM symbol has to be determined before channel estimation can be performed in step 24. As shown in FIG. 2, Transmission Parameters Signaling (TPS) decoding procedure is utilized in step 23 which determines the positions of the SPs by detecting a frame boundary as the scattered pilot positions (known as SPPs hereinafter) are directly related to the OFDM frame. The detection of the frame boundary is so-called “frame synchronization.” Typically, the frame synchronization takes a variable synchronization time of 68˜136 OFDM symbols, 68˜136 TOFDM, which is around 50%˜70% of the overall synchronization time associated with the total synchronization procedure 20. Thus, the conventional frame synchronization is considerably time-consuming. In particular, for DVB-H time-slicing purposes of burst-mode transmission, the receiver may prepare for the required frame synchronization time even longer than the data burst duration of interest. Therefore, the conventional frame boundary detection based SPPs identification (or SPs synchronization) scheme is especially inefficient in the sense of power reduction for receiving the time-sliced DVB-H signals.